Testing of an Improved Cumulus Parameterization in the NMC Global Model

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Dr. Randall has developed a new cumulus cloud parameterization that has been run in a research mode with the general circulation model at Colorado State University. Early results with this parameterization are quite promising. Under this award, the parameterization will be tested in an operational environment using the National Meteorological Center's (NMC) global forecast model. Proper representation of cumulus clouds in numerical models is essential for improving weather prediction. Cumulus clouds are important in releasing latent heat which drives the atmospheric circulation as well as playing a major role in the development and evolution of thunderstorms and tropical cyclones. The new scheme has many features which make it attractive for both practical and scientific reasons. Unlike its predecessor, the Arakawa-Schubert parameterization, this version allows penetrative cumulus clouds to form at any level in the model. It also allows for the formation of shallow cumulus cloud convective downdrafts. The new scheme thus brings the numerical model closer to what is observed in nature. Computationally, the new scheme appears to be superior in terms of simplicity and speed. Forecasts with the new scheme will be compared with those made with NMC's current cumulus scheme. The principal investigator hopes to demonstrate the robustness and model independence of his parameterization. This project is being supported under the NSF-NMC Joint Program in Numerical Weather Prediction.